The present invention relates to a method for eliminating sources of error in the system correction of a coordinate measuring machine.
In general, at least one illumination apparatus is provided for a coordinate measuring machine. Herein, for measuring the position of a structure on a substrate or object (mask for the semiconductor industry, or a wafer), at least one laser interferometer system is used for determining a positional displacement of the substrate or object in at least one spatial direction. The at least one laser interferometer system is accommodated, together with the object or substrate and the entire coordinate measuring machine, in a climate-controlled room.
A measuring device for measuring structures on wafers and on the masks used in their production is disclosed in the lecture manuscript “Pattern Placement Metrology for Mask Making” by Dr. Carola Biasing. The lecture was given during the Semicon Education Program Conference in Geneva on 31 Mar. 1998. The description given therein constitutes the basis of a coordinate measuring machine. With regard to details concerning the functioning and design of this measuring device, reference is made specifically to this publication.
The present invention is advantageously used with a coordinate measuring machine of this type and—without restricting its generality—primarily in conjunction with a coordinate measuring machine of this type. Within the context of the present application, the terms “sample”, “substrate” and the general expression “object” are taken to have the same meaning.
In the production of semiconductor chips which are arranged on wafers, with ever increasing component density, the width of the individual structures becomes ever smaller. Accordingly, the demands placed on the specifications of coordinate measuring machines that are used as measuring and inspection systems for measuring the edges and the position of the structures and for measuring the structure widths are ever increasing. In these measuring devices, it is still optical sensing methods that are favoured, as before, although the measuring accuracy demanded (currently in the region of a few nanometers) lies far below the resolving power achievable with the light wavelengths used (in the near UV spectral region). The advantage of optical measuring devices lies in their substantially less complex design and simpler use compared with systems with other types of sensing—e.g. using X-rays or electron beams.
As a rule, the positions of such structural elements are determined relative to a reference coordinate system of the substrate (mask or wafer) or relative to a coordinate system of the coordinate measuring machine. Together with the position of the measuring table measured by interferometric means, this results in the coordinates of the structure. The structures on wafers or on the masks used for their exposure permit only extremely small tolerances. In order to test these structures, a very high degree of accuracy is therefore always required (currently in the nanometer range). A method and a measuring device for determining the position of such structures is known from the German published application DE 100 47 211 A1. With regard to details of said position determination, reference is therefore made specifically to this document.
The German patent DE 197 34 695 C1 relates to a method for determining a correction function for elimination of coordinate-dependent measuring errors in a coordinate measuring machine through self-calibration. The invention is based on the realisation that there are special components of the correction function which are not unambiguously determined or are subject to very large errors. This mainly concerns components which, in the calibration measurements of all orientations of a reference object used for the calibration, always coincide with themselves (exactly or only approximately), i.e. the rotationally symmetrical components are invariant for the rotations of the reference object that are carried out.
The U.S. Pat. No. 4,583,298 describes the self-calibration of a coordinate measuring machine with the aid of a calibration plate, on which a grid is arranged. The positions of the grid points are not calibrated, however. The grid plate is laid on the object table of the coordinate measuring machine and the positions of its grid points are measured. The same grid plate is then further rotated two or more times through, respectively, 90° about a rotation axis and, in each of the set orientations, the positions of the grid points are measured. The measuring results are mathematically rotated in reverse and various correction factors and tables are optimised so that the reverse rotated data sets have a better agreement. U.S. Pat. No. 4,583,298 concerns itself in detail with the problem of faulty or unreliable corrections. The cause has been identified as being errors in the measuring of the measurement values used for correction determination. It is shown that a mathematically unambiguous correction is only achieved when more than two different rotation positions are measured with the same grid plate. For this purpose, the grid plate is laid, as previously known, on the object table and the positions of its grid points are measured in a plurality of orientations of the grid plate. The orientations are achieved, for example, by multiple rotation through 90° about their mid-point. However, the grid plate must then be displaced to a totally different position on the object table. Once there, the measurement of the positions of its grid points is repeated in a plurality of orientations, as previously known. It is essential herein that the same grid plate must be displaced on the object table.
U.S. Pat. No. 5,798,947 describes a method for self-calibration of tables of a 2-D metrology measuring machine. A plate comprising an N×N grid of marks is used in order to determine the table positions relative to a Cartesian coordinate system. From this a rotation function Gx(x,y) and Gy(x,y) is determined. For the self-calibration, the mask is rotated through respective 90° steps. In addition, measurement with a displaced mask is carried out. Based on the teaching of U.S. Pat. No. 5,798,947, it is not possible to determine error components that provide the same distortion in all the individual measurements.